Smartphones, Tablets and GPS Accuracy

Implementing mobile solutions that target the needs of your field workforce, that leverage your investment in the ArcGIS platform, and are available on the most modern and cost effective device platforms is critical when defining your mobile strategy.

Location is important and depending upon what you are mapping, locational accuracy can make or break a GIS. When considering the use of smartphone and tablet platforms one of the first questions you may ask is – how accurate is the GPS?? The answer to this question will shape the decision you make on device platforms. Or will it??

Cellular-enabled smartphones and tablets include an integrated GPS chip-set manufactured by companies like Qualcomm, Broadcom, CSR and others. These chip-sets are designed first for power consumption, second for time to fix and third for locational accuracy. What we found in our testing is that locational accuracy is pretty much the same regardless of manufacturer. In the open sky, mapping with several devices approximately 90% of all positions collected fell within 3m of our baseline. These results were not too shabby really! In canopy conditions where we encountered significant multi-path interference from the receiver, results were considerably worse.

Most of the chip-sets support GNSS (Global Navigation Satellite System) which means they support both the United States NAVSTAR GPS satellites and the Russian GLONASS satellites. That means you have a better constellation of satellites to draw from when you are in the field.

Next we connected our smartphones to external GPS receivers that were considered “consumer grade”. These chip-sets are designed first for locational accuracy, second for time to fix, and third for power consumption. Here we found that you can purchase a variety of receivers that connect either directly to the device or via Bluetooth and most with SBAS capabilities! In the open sky, virtually all positions captured (99%) were within 3m and almost 70% were within 1m of our baseline! As expected, positions degraded in canopy conditions but overall were much better!

Finally we connected an iPhone 4s to a high-accuracy, dual-frequency, survey-grade receiver from Altus Positioning Systems using a commercial product called GPS Cable from Aman Enterprises Inc. The GPS Cable will create an external connection to virtually any external GPS receiver using RS-232 and stream NMEA to the location manager on your smartphone. This replaces the location source in the same fashion as the external “consumer grade” GPS device does. Not to embarrass the other receivers we tested, and to ensure that we were only using the receiver itself and not supplementing our source using NTRIP (which we could have done), we chose to only map using SBAS and not fully leverage the true capabilities of the receiver. Even so we found that virtually all positions (over 92%) were within 1m!!

A couple of additional points worth mentioning – PPP (Precise Point Positioning) is becoming a reality and with it you can achieve centimeter level positioning directly from a satellite lock! This is a great solution for mapping efforts that require high accuracy, do not want to post-process positions, and do not have access to RTK networks.

So before you consider spending a fortune on mobile devices from hardware vendors that promise sub-meter accuracy with a complicated field to office workflow, consider your field needs and the possibilities of using a smartphone instead! Couple your hardware decisions with the Collector for ArcGIS app and implement a great low cost field data collection solution for your field staff that have not been trained in the underpinnings of high accuracy GPS positioning and want to collect data on a modern device!

To look at some of our tests, check out this web map. Let us know what you think and what you would like to see regarding device testing from Esri.

Once you do a little research, you’ll be surprised which GPS units do and don’t perform in adverse conditions (next to buildings or under tree canopy). Sometimes lower cost units do better than price ones, but this is hard to establish. First ask if data use justifies the expense for high accuracy. Sometimes one really wants consistency, the ability to get a good lock, quick and with reasonable accuracy. Chasing the last centimeter of accuracy may not be necessary.

This is a great point! Quite often field collection projects do not require high accuracy positions. In fact quite often field work involves updating attribute information of existing feature data or redlining locations with an approximate location that drives additional field work from other business units. Consider the spatial accuracy need before choosing a device and/or GPS receiver.

Thanks for the tips, and for the recommendations of external GPS devices that are compatible with smartphones. I was glad to hear from the Collector dev team just this morning that disconnected editing, as well as support for lines and polygons, will be available in the next release. Combined with the mobile platform SDKs, users should be able to create some pretty powerful data collection apps.

We were testing with the APS-3 (http://www.altus-ps.com/aps-3/the-aps-3.html) but hope to get our hands on the 3L later this summer. We were using the GPS Cable (30pin on iPhone 4s to RS-232 connection on the 3). No USB…

hi, appreciate this wonderful post and information, sharing your web map can help me explain easily to my users about GPS accuracy considerations.
May I ask have your team considered indoor positioning ? Other than using Google’s location provider, do we have other choices using collector app for indoor use ?
-Andy Tan

Indoor positioning is a very interesting topic and we are keeping a watchful eye on the technology. There are a few competing technologies in this space and it is a growing trend. If you have not had a chance to look at our offerings with Facilities GIS, please take a look here (http://resources.arcgis.com/en/communities/facilities/). This is a growing space for GIS and Esri.

This topic seems to be very hot right now. We have been suffering from countless user complaints with the new GeoXTs we purchased, and have been considering building custom data collection apps and have been looking for ways to pair the devices with external GPS antennas. We have an old Trimble XB receiver that I was able to prove was overriding the internal GPS antenna of the phone. There are still so many questions, but I think with 10.2 releasing improved offline capabilities and the collector app supporting offline, will really change how we collect data in the field. Goodbye to the days of Trimble poor user experiences and hello to to the awesomeness of pain free iPhone data collection.

If you guys could do a whitepaper or something on this topic, it would be greatly appreciated.

While I agree with the broad thrust of this article, the real test of GPS accuracy is what we get in an offline disconnected mode – and the accuracy there is substantially reduced than what is claimed here.

However, efforts are underway to build improved GPS accuracy into tablets and we too are shortly expected to come out with a solution for this.

We will be doing this level of testing soon and I will make sure to publish our reports. You should expect that when using the internal GPS chip accuracy will degrade when there is no connectivity (positioning is assisted by geo-located wifi and cell). However when you use an external GPS receiver, connectivity does not matter. There are a lot of consumer grade GPS devices that you can use as I mentioned above. We tested the Bad Elf GPS (http://bad-elf.com/products/be-gps-1000) fully disconnected with pretty good results. Stay tuned and we will publish offline results this fall.

We use iPads in disconnected (off-network) mode for data collected all the time. I haven’t noticed any loss in accuracy vs. the GPS when on the cell network.

What I have noticed is that it takes longer to “settle” and reacquire a good position after say, hopping a few miles downstream in a helicopter. Even then – it usually is back in business in about 8 seconds.

As far as I can tell the “aGPS” helps with the first stages of GPS satellite aquisition (provides an almanac and a head-start)… but in the end it all seems to go back to antenna. Ipads have a larger antenna area than iPhones – and seem to have a slightly more reliable position. Bad Elfs (Elves?) also have consumer grade chips, but aren’t surrounded by a metal enclosure and have a better antenna.

We are hoping to rig up a bluetooth connection to an SXBlue GPS unit sometime in the near future – to test the feasibility of pulling in sub-meter signal to our SDK based iOS apps.

Mike – thanks for posting your comment. I was thinking specifically about positions degrading under urban canopy when disconnected. What we have observed is that sometimes assisted fix can provide more reliable positions that the GPS chipset!

Genec with the SxBlue series of receivers have been very successful. For others that have not yet looked at them yet, you really should! http://sxbluegps.com/ They have some exciting new technology coming soon…

You need to also consider the application APIs on the smartphone and tablets. Microsoft’s location API for instance combines readings from the gyroscope, GPS, and motion sensors to provide a combined computed average of location and orientation. Adding an external GPS adds to the accuracy but it is important to note that the API does not read solely from that external GPS.

Very interesting article. The deployment of the new Block III GPS satellites will be a game changer. The new L1C, L2C, and L5 signals will allow much greater accuracy from just about any mobile device in the future. These new signals will have increased power (current GPS signals are only broadcast at 50 Watts or so) which allow it to reach in places it has not in the past and reduce atmospheric interference. The L5 and L2C will also include an expanded NAV message which will also allow receivers to better determine their position faster and more accurately.

If you add in the China’s new GNSS called Compass and the Galileo GNSS from the European Union, then we will have even more satellites which can be used for calculating locations. More satellites means better locations. Galileo was designed from the beginning to be compatible with GPS. It will also be using L5 (They refer to it as E5a) and broadcasting the expanded NAV message. This should make it easier to adapt existing GNSS receivers to make use of the Galileo constellation as it comes online. As of today, there are 4 Galileo satellites in orbit and the first Galileo only position was captured this year. Current plans call for a fully operational 24 satellite constellation by 2016.

Our priorities for a chip-sets may be a little different: first for time to fix, second for power consumption, and third for locational accuracy. Are there any resources out there that evaluate time to fix differences in smartphone chip-sets? Are they all equal? It looks like we could better accuracy with an external GPS receiver, but would an external GPS improve time to fix, or would it be about the same as the internal gps?

Our mobile data collection efforts typically don’t have access to a newtork or wifi. Asking field workers in remote parts of Africa to wait 10-15 minutes for an initial lock is a tough sell, they are more likely to give up.

I haven’t had much luck finding resources on differences in consumer level smartphone GPS chip-sets in this regard, or even which phones use which chip-sets.

Do differences in the various mobile operating systems give the developer more or less control and/or constraints in interpreting data from the GPS? For example, for iOS, do parameters such as stationary speed/distance thresholds come into play in the Collector for ArcGIS app? I notice that when using Collector for iOS and I’m perfectly stationary, there is absolutely no variation in my series of collected coordinates. Such precision/repeatability seems too good to be true.

http://amanenterprises.com provides the GPS cable and have a NTRIP client built on iOS that runs in the background once started and continues to feed the corrections to the GPS while in background while users can collect data on the collector App…

Remarkable. GPS has become a vital application to most of the modern gadgets. Port-Elec Limited has launched some Smart phone and tablets which are represented in a advanced way. But the excessive number of users has become the reason of slow processing and this will have to be overcome soon…………